NEW YORKERS & CO.; Big, Beautiful Numbers: 820, 834, 960

NANCY RICHARDSON, a longtime Fifth Avenue resident, calls it a ''juste milieu.'' That, she says, is ''a silly French phrase'' that means finding the right tone or happy medium for her way of life.

To some, a happy medium can be found at 960, 834, 820, 740 and 720. For some, these are meaningless numbers; for others, they constitute a code -- what Tiffany stationers call ''beautiful addresses.'' They are the addresses of Fifth and Park Avenue apartment houses -- A-plus buildings -- that signify that you are wealthy and social, that you have made it to the pinnacle of what many consider world society.

To live at one of these addresses is ''a privilege and congenial way of life,'' said Ms. Richardson, who was recently divorced from the financier Frank Richardson.

What exactly is an A-plus building? And what happens in them at the height of a real estate boom, when the stock market has multiplied people's wealth enormously? A new genre of apartment-seeker bidding into ''the teen millions'' (as real estate brokers put it) for homes like Ms. Richardson's is desperately trying to get behind the velvet rope. But supply and demand is making it very difficult.

''It's the same amount of product,'' said Louise Beit, a vice president at Stribling & Associates. ''But there are more customers than ever. More than even the height of the 80's.'' Standing in apartment at 820 Fifth Avenue, surrounded by paintings of French landscapes, she said that about one or two apartments come available each year.

The facts of life are these: Even when apartments do come up, sellers first turn to their own friends (they are rarely advertised). Co-op boards are feeling more powerful than ever, rejecting people with $60 million to $100 million in assets. It takes the veto of only one person you've crossed. (The chairman of Revlon, Ron Perelman, did not make it into 820 Fifth Avenue.)

A list of A-plus buildings circulates among what the primary gatekeepers -- so-called social brokers -- who size up clients before showing an apartment. ''I call their bank officer,'' said Hall Willkie of Brown Harris Stevens, a real estate firm. ''That is, of course, if they have a bank officer.''

What are the criteria for getting into an A-plus building? Payment in full in cash; impeccable social credentials (private clubs, schools and the location of your summer home counts); substantial philanthropic contributions (tithing scores well) and having $60 million to $500 million in assets, a substantial portion liquid.

Rest easy. Buildings on the East River require a bit less. (There are ways, with a bank's help, to arrange your finances so that co-op boards do not know the source of your cash.)

What goes into the rating of an A-plus building? First, location: all of the Upper East Side, especially Fifth and Park Avenues between 60th and 79th Streets, and the East River. West Side apartments do not make the list, although devotees of the Dakota, Beresford and San Remo consider those buildings A-plus.

Then there is the ''quality'' of tenants like Lawrence Rockefeller, Ann Bass and Cynthia Phipps. And, finally, there's the architecture: the magic name is Rosario Candela, a 1920's architect known for grand flowing apartment layouts.

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Most A-plus apartments have 8 to 20 rooms and features like wine cellars, walk-in vaults, linen rooms the size of studio apartments and extensive servants' quarters.

There is a certain irony at play. In the early 1900's, when some of these buildings were built, they were closed to anyone who was not born or did not marry into the right families. But economic ups and downs opened the doors. Now some of the very people who bought floor-through Park Avenue apartments for perhaps $80,000 in the early 1970's are trying to close the doors, even tighter than before.

Why? Exclusivity sells. ''Buildings are making a conscious choice to become more unreasonable so they will appear more exclusive,'' said Frederick W. Peters, president of Ashforth Warburg Associates and a co-chairman of the board of the residential division at the Real Estate Board of New York. ''It's really infuriating.''

But it is working. But this time tenacious buyers, with enough financing, are trying even harder than their predecessors to climb in. ''I tell people they are not going to make it past the co-op board, but they want to try anyway,'' said Sharon E. Baum, director of the exclusive property division at the Corcoran Group, sitting in her Rolls-Royce on her way to show an apartment at 19 East 72d Street. ''Before, people were embarrassed if they didn't pass. Now when they get turned down, they want to try again.'' She has had two board decisions reversed.

There are people who do not want to live on Park and Fifth Avenues because they find the screaming status of the address like wearing a label on the outside of a dress. They prefer to live in what are considered more discreet social buildings, like 1 Beekman Place, 1 Sutton Place South or even 19 East 72d Street (the building famous for turning down Richard Nixon, though it is rated A-minus because it is on a side street).

''It's a great way of life: You go home and never feel like you're in New York -- just the wind off the river,'' said Juliet Hartford, 27, the daughter of Huntington Hartford, the A.& P. heir, who lived at 1 Beekman Place for 35 years, later selling the duplex to Philip Niarchos, the son of the late Greek shipping magnate Stavros S. Niarchos. That high-luxe building has a swimming pool, gardens and private rooms at its base, all overlooking the East River, and apartments with servants' quarters the size of some Connecticut homes.

Deluxe apartments on Fifth Avenue were started in 1911, at 998 Fifth Avenue at 81st Street, according to Andrew Alpern, the author of ''Luxury Apartment Houses of Manhattan'' (Dover Publications, 1992). Builders, he said, ''Touted the fact that it was more convenient to live in an apartment because you could very quickly close up shop and go off to your country place or Europe. When you came back, the service people in the building would get the apartment ready for you.''

The builders and architects also billed apartments -- mansions stacked on top of each other -- as more practical, secure and even a more economical way to live than in separate homes.

When Ms. Richardson was looking for a place, she said, ''I just wanted as nice things as we could afford put in a cozy setting, and this was the best value for the space.'' She added, ''The market was nothing like it is now.'' And, she said, ''While these buildings have this reputation of maybe being exclusive, it's really all about who would make a congenial neighbor and getting enough of space.''

The great apartments are truly palatial, with sweeping entrance ways downstairs and upstairs, endless numbers of rooms and museum-quality design detail on the ceilings, windowsills and doorknobs. The Picassos and Renoirs on the walls do not hurt the ambiance. Still, even then some lives within those walls do not always find that juste milieu.